The present invention relates to a system for projecting a light spot of variable intensity on a workpiece and more particularly to a system for varying the intensity of the light spot as different sized spots are projected onto a workpiece having a photosensitive work surface and for varying the intensity of the light spot as the spot is moved at velocities less than maximum to uniformly expose upon the photosenstive work surface various lines of different thicknesses in a minimal amount of time.
In forming graphic information on a photosensitive surface, line thicknesses inscribed on the surface may be varied according to the pattern of information by selectively positioning different size apertures in registry with a light beam to generate a single spot of light on the photosensitive surface. A line is generated on the photosensitive surface by the relative movement of the light spot and the photosensitive surface. It is desirable to evenly and uniformly expose different line thicknesses generated by the different aperture sizes as graphic information on the photosensitive surface. Since, for equal speeds of movement, a larger diameter light spot delivers radiant energy over a longer period of time when moved over a given incremental area on the work surface than a small diameter light spot, to obtain equal exposures the intensity required for the smaller aperture size must be greater than that of the larger aperture size. Where insufficient light is projected from the lamp, the line attempted to be created by the spot will be a hazy underexposed image. Conversely, if the amount of radiant energy projected onto the photosensitive work surface is too great, diffusion and scattering of the light beam will take place to undesirably darken unintended areas of the film to create, at best, a fuzzy undefined line on the photosensitive work surface. Thus, the intensity of the light spot projected onto the photosensitive work surface must therefore be varied to compensate for the aperture size selected.
Also, it has been found that as the light spot is moved relative to the photosensitive work surface at varying velocities, the intensity of the light spot must also be varied proportionally with the change in velocity to obtain uniform and even exposure of the lines defining the graphic information. When the light spot is moved slowly relative to the photosensitive work surface, the spot is slowly exposed projected across a given incremental area on the photosensitive work surface. When however, the light spot is moved at a faster velocity relative to the photosensitive work surface, a greater spot intensity must be generated to compensate for the shorter time that the spot is projected onto the photosensitive work surface. Accordingly, to achieve uniform exposure of the photosensitive work surface along an entire line length, the light spot intensity must be varied to generate a significantly greater light spot intensity when the light spot is moved at a faster velocity than when moved at a slower velocity.
U.S. Pat. No. 3,330,182 issued July 11, 1967 addresses the above mentioned drawbacks when exposing a light spot on a photosensitive surface. The patent employs individual filters corresponding with individual aperture sizes formed in an aperture wheel and further varies the intensity of the light beam as the beam is moved relative to a photosensitive work surface. By either rotating a variable density filter disc through the light beam or by varying the lamp illumination voltage proportionally to the speed of the spot, variable spot intensity is accomplished. However, the patent is not concerned with moving the light spot whenever possible to attain a light spot maximum velocity whenever a line is created by the relative movement of the spot and the photosensitive work surface. Also, the patent does not assign different lamp voltages to correspond with the different spot sizes to achieve uniform intensity of any size spot when any selected spot is moved at the maximum velocity. Movement of the light spot relative to the work surface at a maximum velocity is important and is a critical time saving feature when successive lines are created on the photosensitive work surface in the formation of today's complex graphic information such as found in circuit patterns. Lamp usage is accordingly minimized when graphic information is produced rapidly as a result of the light spot being constantly moved to attain the system maximum velocity. Thus, by creating graphic information with movements occurring at the maximum system velocity, longer lamp life is effected and less heat is generated by the lamp during the process. Also, as the lamp filament employed in U.S. Pat. No. 3,330,182 becomes worn with use, the illumination characteristics of the lamp may become unpredictable. Since the patent provides no means by which the emitted intensity may be checked and compensated for, creation of lines defining precise graphic information on the photosensitive work surface by the lamp may be inaccurate.
Accordingly, it is an object of the present invention to provide a system for projecting a light spot on a photosensitive surface which is moved to attain a light spot maximum velocity whenever possible to expose on a photosensitive work surface lines creating different patterns of graphic information in a miminum amount of time.
It is another object of the present invention to designate different lamp command voltages each corresponding with a different light spot size and each lamp command voltage selectively energizing a lamp to project a light spot having a uniform intensity relative to any of the other different light spot sizes when any one of the different sized light spots is moved at the maximum velocity.
It is yet another object of the present invention to utilize the actual velocity of the light spot as it moves relative to the photosensitive surface to alter any one of the designated different lamp command voltages corresponding to the different light spot sizes proportionally with respect to the actual velocity value to thereby achieve an even exposure of a line by the light spot when the light spot is moved at velocities less than maximum regardless of the selected light spot size.
Still another object of the present invention is to provide a lamp intensity feedback system sensing the actual intensity of the lamp and correcting the lamp intensity in accordance with the intensity corresponding to a selected one of the designated different lamp command voltages.
Yet a further object of the present invention is to provide a system moving a light spot to constantly attain a light spot maximum velocity whenever possible to reduce the illumination time of the lamp thus reducing heat generated by the lamp and accordingly increasing lamp life.